The present invention relates to imaging apparatus and methods, and more particularly, to image capture apparatus and methods of operation therefor.
There has been tremendous growth in multimedia applications for personal computers. For example, many types of peripheral multimedia devices for processing video and audio data are now available for use with personal computers, including video devices that capture image data for further processing by a personal computer. The image data may represent a still image or a sequence of images.
In a typical solid-state image capture device such as a charge-coupled device (CCD), an array of image capture elements receive an optical signal, i.e., light radiating from an object, and store an amount of charge corresponding to the received optical signal. Vertical and horizontal transfer units output video signals corresponding to the stored charge according to horizontal and vertical period signals. A timing signal generator typically generates various signals related to operation of the image capture device, such that the video signals are output by the two-dimensional solid-state imaging device for each of a succession of xe2x80x9cfields,xe2x80x9d such as the {fraction (1/60)} second fields defined under the NTSC (National Television Standards Committee) standard and {fraction (1/50)} second fields defined under the PAL (Phase Alternating Line) standard. A xe2x80x9cframexe2x80x9d typically represents a time interleaving of two fields. Accordingly, a frame is typically produced every {fraction (1/30)} sec for NTSC systems and every {fraction (1/25)} sec for PAL systems.
Referring to FIG. 1, video data is output from a image capture device such as a CCD every {fraction (1/60)} sec for NTSC and {fraction (1/50)} sec for PAL. A vertical drive signal VD operates as a reference for the read operation, and signals XSG1 and XSG2 represent gate signals for transferring charge from the image capture device to a vertical transfer unit. Charge previously transferred to the vertical transfer unit is then transferred to a horizontal transfer unit line by line, and a video signal corresponding to the output image data is produced by the horizontal transfer unit. To provide exposure control, a shutter pulse is provided to discharge image capture elements in the CCD for a shutter interval to control a charging period of the image capture elements of the device. These and other basic operations of image capture devices are well-known to those skilled in the art.
In conventional systems, video data produced by such an image capture device may be processed at various rates. For example, some applications process video signals at rates that vary from 7.5 frames per second to 30 frames per second. Because data is typically read out from the image capture device at a fixed field rate, a buffer is typically used to store fields of video data read from an image capture device so that they may be processed at variable frame rates.
In light of the foregoing, it is an object of the present invention to provide an image capture apparatus and methods for operating an image capture device at varying frame rates.
It is a another object of the present invention to provide an image capture apparatus and methods that capture image data at varying frame rates without requiring buffering.
These and other objects, features and advantages are provided according to the present invention by applying gate signals to an image capture device for selected fields of a succession of fields responsive to a commanded frame rate, and shuttering the image capture device responsive to the commanded frame rate. Preferably, the commanded frame rate is selected such that the field rate is an integer multiple of the commanded frame rate. In one embodiment, gate signals are generated for each of a succession of fields, but are only applied in selected fields based on the commanded frame rate through the action of a gate enable signal.
In particular, according to the present invention, image capture data is output from an image capture device, e.g., a CCD, at a commanded frame rate. A succession of fields that occur at a field rate are identified. A commanded frame rate that is less than or equal to the field rate is received. Read signals are applied to the image capture device for selected fields of the succession of fields to thereby output image data at the commanded frame rate. The image capture device is shuttered for a shutter interval based on the commanded frame rate to prevent image capture during the shutter interval.
According to aspects of the present invention, the commanded frame is selected such that the field rate is an integer multiple of the commanded frame rate. In one embodiment application of read signals to the image capture device is preceded by the step of generating a count of fields. Read signals are applied to the image capture device responsive to the count of fields meeting a criterion related to the commanded frame rate, e.g., to the count of field equaling the number of fields in a frame corresponding to the commanded frame rate.
In another embodiment, a read signal is generated for each of the succession of fields. The generated read signals for selected fields are applied to the image capture device through, for example, the action of a gate enable signal that is generated based on the commanded frame rate.
An image capture apparatus according to the present invention includes an image capture device, e.g., a CCD, configured to receive read signals and operative to output image data responsive thereto. A timing generator is operative to produce a timing signal that defines a succession of fields that occur at a field rate. A variable frame rate image capture controller is coupled to the timing generator, configured to receive a commanded frame rate, and operative to apply read signals to the image capture device for selected fields of the succession of fields to thereby output image data from the image capture device at the commanded frame rate, and to shutter the image capture device during a shutter interval determined based on the commanded frame rate.